CRUSTAL MOVEMENTS ON SHIKOKU, SOUTHWESTERN JAPAN, INFERRED FROM INVERSION ANALYSIS OF LEVELING DATA USING ABIC

With an inversion technique based on Akaike's Bayesian Information Cri terion (ABIC) we analyzed levelling data for 1893-1983 on Shikoku, sou thwestern Japan, where large interplate earthquakes have periodically occurred at intervals of about 120 years, releasing tectonic stress pr oduced by steady relative motion of the Philippine Sea and the Eurasia n plates, Through the inversion analysis we reconstructed the pattern of crustal movements on Shikoku over the last 120 years, including the occurrence of the 1946 Nankaido earthquake (M 8.1). The result clearl y shows that the crustal movements on Shikoku include significant secu lar vertical motion (uplift in the south and subsidence in the north) in addition to cyclic motion related to the periodic occurrence of int erplate earthquakes at the Nankai trough. Contrary to the widely accep ted theory, we could not find any correlation between the secular vert ical motion and the coseismic vertical displacement. The secular uplif t motion on southern Shikoku estimated from the levelling data complet ely agrees with that inferred from the present heights of marine terra ces formed by eustatic sea level changes and crustal uplift for the la st 10(5) yr. This suggests that the fundamental causes of the short-te rm (10(2) yr) and long-term (10(5) yr) movements on southern Shikoku a re the same, the steady subduction of the Philippine Sea plate at the Nankai trough. On northern Shikoku, on the other hand, the pattern of secular crustal motion estimated from the levelling data is quite diff erent from that of the Quaternary uplift inferred from the present hei ghts of eroded flat surfaces, indicating the existence of some unknown tectonic process controlling the very long-term (10(6) yr) crustal mo vements.